1. Field of the Invention
The present invention relates to the packaging of semiconductor devices. More particularly, this invention relates to the packaging of power transistors to form a power supply.
2. Description of Related Art
With the advent of microprocessors and other electronics that consume more electrical power, comes the need for higher output power supplies. While microprocessor voltage requirements are decreasing, substantial increases in electric current requirements are offsetting this trend. Future processors may consume current at levels an order of magnitude higher than today's processors, and will dissipate much more power because the operating voltage will not decrease proportionately. GHz class central processing units (CPUs) are routinely specified to draw 30 A or more of direct current. Therefore a need has arisen for high current, low voltage power sources which have a small physical size and generate a modest amount of heat. (As is conventional, “power source” or “power supply” refers here to an electrical voltage/current converter, not to the ultimate source of the electricity, such as a battery or generator.)
Heat sinking is important for the power semiconductor devices which form the primary components of the power supply systems. Power semiconductor devices tend to generate substantial amounts of heat, because they are required to switch large currents. Furthermore, the power semiconductor devices are often deployed in environments that are relatively hot, such as in an engine compartment or in the chassis of a personal computer. The dissipation of heat generated by the power semiconductor devices is accomplished by, for example, mounting the chip to a thermal heat conducting flange and then mounting the heat conducting flange to a bracket with fins formed therein and exposing the finned structure to a cooling air flow.
However with the miniaturization of electronics products, less room exists within the power supply packages for dissipating the heat generated by the devices. Furthermore, the miniaturization has driven the reduction in size of the integrated circuits (ICs) or discrete semiconductor devices such as transistors (e.g., field effect transistors FETs), and an increase in packing densities of the ICs or FETs on the power supply circuit board. The amount of heat generated per unit area by the reduced footprint (size) ICs or FETs increases with their packing density. The requirements for packaging these heat-generating ICs or FETs in ever smaller areas, make the need to provide adequate heat exhaust paths more urgent, and this is particularly so for high current devices such as power supply modules.
Therefore, because of the continued trend to miniaturize electronic products, the power supplies that power the products are required to become smaller, and particularly to become thinner. The thickness of the power supply may be dictated in part, by the connector pitch on the motherboard, which may be for example 0.6 inches. Therefore, the thickness of the power supply package may be preferably less than 15.24 mm. Because of the small size of the packages, large thermal masses are no longer available to dissipate heat. Furthermore, little space remains between the packages, in which to circulate the cooling fluid such as air.
Therefore a problem remains in the design of high current power supplies, in sourcing and sinking the large currents but keeping the physical size small, the cost low and limiting the temperature rise.